Control circuit for electrodeposition system using silicon controlling rectifiers



Feb. 17, 197% D. E. eoonwm 3,495,037

CONTROL CIRCUIT FOR ELECTRODEPOSITION SYSTEM USING SILICON CONTROLLING RECTIFIERS Filed Mme 21, 1966 "M" SLOPE ADJ UST INVENTOR.

United States Patent CONTROL CIRCUIT FOR ELECTRODEPO- SITION SYSTEM USING SILICON CON- TROLLING RECTIFIERS David E. Goodwin, Middletown, N.J., assignor to M & T Chemicals Inc., New York, N.Y., a corporation of Delaware Filed June 21, 1966, Ser. No. 559,252 Int. Cl. B01k 3/00 US. Cl. 204-228 6 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus are provided for a silicon controlled rectifier power supply for automatically regulating the output of the silicon controlled rectifier power supply through the source of power therefor from a pre-set low output to a pre-set high output during a predetermined time interval and including control of the time intervals of the low output and the high output levels. A gating circuit is included also for supplying the silicon controlled rectifier the source of power for which includes an amplifier-capacitor arrangement with a variable resistance for the said pre-set low output, and an integrator arrangement formed by said amplifier-capacitor connected alternately to a resistance and a variable resistance and including a potentiometer for said high output. Further, a time relay is provided for interconnect ing said amplifier-capacitor arrangement alternately to provide the pre-set low output and the pre-set high output.

This invention relates to a control circuit for controlling a silicon controlled rectifier power supply (hereafter referred to as an SCR power supply) system used in electrodeposition, and particularly to a circuit for controlling an SCR power supply to provide plating current which is initially of a low magnitude and which increases over a predetermined time interval toward the desired full plating current value. The full plating current value is then maintained for as long as is desired.

In certain electrodeposition processes, particularly metal plating processes, it is necessary to provide a low initial plating current, after which a current is increased at a predetermined rate until the full plating current value is achieved. Full plating current is then maintained for as long a period as is desired. It has been found, for instance, that the white blotchy patches obtained when plating chromium by conventional methods are avoided when the plating current is increased from an initially low value in this manner. In carrying out this type of process, a voltage is ordinarily applied between the object to be plated and the anode before such object is placed in the plating bath, This initial voltage activates the surface of the object to be plated and should be sufficient to provide a current of only approximately onetenth the desired full plating current. After the object is immersed in the plating bath, the current is increased gradually to the desired value and maintained until the required plating thickness is achieved. The interval during which the current increases from its initial to its final value is on the order of between two to sixty seconds, and usually from ten to thirty seconds. The full plating current may then be applied for typically between one and 10 minutes.

It is desirable that the plating current, which is ordinarily provided by an SCR power supply be automatically controlled so that uniform results may be obtained in an efficient manner. Most applications require that the initial low voltage be maintained from before the object to be plated is put in the plating bath until after it is fully immersed. Since this time will vary, depending on the particular plating facility used, it should be easily variable. Moreover, the period during which full plating current is to be applied will vary according to the object to be plated, the particular bath used, the full plating current value and the desired plating thickness. This time must therefore also be conveniently adjustable. Finally, the interval during which the plating current is increased from its initial value to the full strike value must be adjustable, Furthermore, this increase in plating current must be achieved over a period longer than most elec trical time constants, and is preferably monotonic.

Accordingly, the present invention provides a control circuit for automatically controlling the current output of SCR power supply, whereby said current is caused to rise from an initial current magnitude to a higher current magnitude during a first predetermined time period and is caused to maintain said higher current magnitude for a second predetermined time period. A particular embodiment of such a control circuit is described herein in which an initial voltage of low magnitude is provided, which voltage is gradually linearly increased to the full plating value and maintained for the desired interval.

The invention will be more fully described with respect to the figures of the accompanying drawings, in which:

FIGURE 1 is a schematic diagram of a control circuit according to the present invention; and

FIGURE 2 is a plot of control circuit output voltage versus time for a typical plating cycle.

Referring to FIGURE 1, an embodiment of the invention is illustrated in which a high gain D.C. amplifier 10 is connected in parallel with the capacitor 11 to form an integrator, or integrating amplifier. Initially, the amplifier 10 performs as an amplifier, amplifying a constant input voltage to provide a constant output voltage E with a value E indicated in FIGURE 2 as the output before time T The value of the voltage E may typically be about from 2 to 5 volts. The input voltage prior to T is applied to the input of the amplifier 10 through a voltage divider network which includes a resistor 12 and a variable resistor 13. During this time relay 14 is in its actuated state, connecting relay terminals 15 and 16. In this position, the resistor 13 also acts to provide negative feedback around the amplifier 10. Typically, if the value of the resistor 13 is substantially smaller than that of the resistor 12 (about one-eighth the value of the resistor 12) the output E is approximately R /R V In this state, the amplifier input is held slightly negative at substantially zero volts, and the output voltage E may be generally around two volts, assuming approximately the above-mentioned ratio R to R and a power supply voltage V of about 16 volts. For example, the amplifier output voltage will be approximately 2 volts with a R to R ratio of 1 to 8.

The setting of the variable resistor 13 determines the output value E of the amplifier 10, and this resistor should be chosen to afford the desired adjustable range of E for the particular installation.

At time T the relay 14 is deactuated connecting the relay terminals 16 and 18, which puts a negative voltage step across the input of the amplifier 10. Deactuation of the relay 14 may be achieved by any convenient means, such as a timer, or manually; whatever signal causes deactuation will be referred to herein as a trigger signal. The integral of the last mentioned input voltage step, which is a ramp function, appears at the output of the integrator formed by the amplifier 10 and the capacitor 11. The slope of this ramp function is determined by the values of the capacitor 11, the resistor 19, and the variable resistor 20. If the variable resistor 20- has a value of about 100K, the resistor R19 is about 1K and the capacitor 11 has a value of about 250 t, variation of the resistor 20 over its entire range allows a variation in the output voltage rise time of from about one to about 30 seconds. The value of the variable resistor 20 should thus bechosen to provide the desired range of slopes in the output ramp voltage, and thus the desired range of rise times. The plot of FIGURE 2 illustrates the variation in output wave forms obtainable by varying the resistor 20, and indicates how the slope of the ramp voltage determines its rise time.

When the amplifier output E reaches the preset voltage E determined by the setting of the potentiometer 2 1, the diode 17 becomes a short circuit and the output E is clamped at the predetermined voltage E The full plating voltage, or that voltage corresponding to full plating current, will be determined by the value of the voltage E and will thus be a function of the voltage V and the setting of the potentiometer 21. V may conveniently be between about and volts, and the resistance of the potentiometer 21 may be on the order of a few hundred ohms.

Direct current for the plating process is provided by an SCR power supply 22. Conveniently a three-phase rectifier circuit providing full wave rectification may be used. The power supply 22 is conventionally controlled by an SCR gate drive circuit 23. SCR gate drive circuits are Well known as regulators of SCR power supplies. They are of several types, but all basically control the firing angles of the individual SCR devices to provide the desired overall output level from the SCR power supply. In particular, SCR gate drive circuits may contain saturable reactors which control the last-mentioned firing angles according to the current flowing in the DC windings of the reactors. Such D-C windings are indicated schematically in gate drive 23, but for present purposes it is only necessary to consider a control terminal pair, as control input to the SCR gate drive, the current through which determines the output level of the SCR power supply controlled thereby. The SCR gate drive 23 controls the SCR power supply 22 through a control channel 24.

In the circuit of FIGURE 1 the SCR gate drive 23 functions in the manner of a differential amplifier, comparing the output voltage of the SCR power supply 22 with the desired control voltage E at the output of the amplifier 10. Any difference between the desired control voltage E and the SCR power supply output voltage causes a compensating current to flow in the D-C reactor windings (control windings) of the SCR gate drive. This in turn corrects the SCR power supply output volt age in a direction such that the current in the D-C reactor windings is minimized. Thus the SCR power supply output voltage follows the control voltaage E. The SCR power supply output voltage always remains slightly below the control voltage E, so that a small control current always flows in the control windings of the SCR gate drive. The value of this current is always that necessary to make the SCR power supply output voltage follow E. A diode 25 may be provided between the amplifier output and the SCR gate drive to prevent current flow from the latter to the former, since in some SCR gate drives this is an unstable condition. It will be apparent that if the SCR power supply output voltage falls too far below the control voltage E, the control current will increase, causing the SCR power supply output voltage to rise. If the SCR power supply output voltage rises to a level too near the control voltage E, the current in the control windings of the SCR gate drive falls, causing the SCR power supply voltage to fall. Thus the SCR power supply output voltage follows the control voltage E; in practice, the difierence between them is only about a few tenths of a volt. After the desired plating time has elapsed, the relay 14 may again be actuated,

causing the output voltage E and thus the SCR power supply output voltage to return to E It will be understood that the foregoing description relates to a particular embodiment or embodiments only, and is therefore merely representative. In order to fully appreciate the spirit and scope of the invention, reference should be made to the appended claims.

I claim:

1. In an electrodeposition system wherein plating current is provided by an SCR power supply, the combination which comprises an SCR gate drive for determining the plating current supplied by said SCR power supply, control circuit means for automatically causing the SCR power supply current to rise from an initial current magnitude to a higher current magnitude during one time period and for maintaining said higher current magnitude for a subsequent time period, said control circuit means providing an output voltage which increases monotonically during said one period from an initial voltage corresponding to said initial current magnitude to a voltage corresponding to said higher current magnitude and remains substantially constant at said value during said subsequent time period, and said control circuit means including means for providing a substantially constant output voltage at said initial voltage value during a first time period;

means responsive to a trigger signal for monotonically increasing the output voltage from said initial value during a second time period; means for maintaining the output voltage substantially constant at a value corresponding to said higher current magnitude during a third time period; and

said SCR gate drive including a control terminal pair, said system including means for applying said output voltage to one terminal of said terminal pair and the SCR power supply output voltage to the other terminal of said terminal pair so that the difference between said output voltage and the SCR power supply output voltage appearing across said control terminal pair causes the SCR power supply output voltage to follow said output voltage. 2. Control circuit means as defined in claim 1, including an integrating amplifier for providing said output voltage;

input means for providing a first input voltage at the input of said integrating amplifier during a first time period and responsive to a trigger signal for providing a second input voltage at the input of said integrating amplifier during a second time period; and

means for clamping said output voltage at the value corresponding to said higher current magnitude during a third time period.

3. Control circuit means as defined in claim 2 wherein said SCR gate drive includes a control terminal pair, said system including means for applying 'said output voltage to one terminal of said control terminal pair and the SCR power supply output voltage to the other terminal of said control terminal pair so that the difference between said output voltage and said SCR power supply output voltage appears across said control terminal pair.

4. Control circuit means as defined in claim 3 including a diode between said integrating amplifier output and the SCR gate drive control terminal to which said output is applied for preventing current flow from the SCR gate drive to the amplifier output.

5. Control circuit means as defined in claim 4, including means providing resistive feedback around said integrating amplifier during said first time period and responsive to said trigger signal for removing said resistive feedback during the second and third time periods.

6. Control circuit means as defined in claim 2, including means for adjusting the first input voltage provided by said input means,

means for adjusting the length of said second time period, and said output voltage clamping means 3,496,087 5 6 being adjustable to vary the value of the output FOREIGN PATENTS voltage during said time period. 1 Great i i References Cited 1,425,475 12/ 1965 France.

UNITED STATES PATENTS 5 JOHN H. MACK, Primary Examiner 3, 1, /1 La ChapeHe 204-228 D. R. VALENTINE, Assistant Examiner 3,377,542 4/1968 Glorioso 323 22 XR 3,383,303 5/1968 Fenoglio et a1. 204-223 US. 01. X.R.

3,386,024 5/1968 Koltuniak et a1. 32118 XR 32118; 32322 

